Feature/private pages (PR 579 rebased)

[orel1]

Hello, The feature allowing private pages is interesting. I simply rebased the PR #579 commits and all works! In response to the initial pull request, I've added some documentation on how it works. For the sake of completeness, it would be preferable for these private pages to be filtered in the pages "All pages" or "Recent activity", but I'm unable to do that in haskell :cry: Do you think this feature could be merged as it stands?